The layered film in which a retardation film (corresponding to optical compensation method) is layered on a polarizing plate via an adhesive layer is known as a layered film of the present invention. A layered configuration example of the layered film is shown in FIG. 12. A layered film 11 is configured by a polarizing plate 1 including a polarizer 1a and a protective film 1b layered on both sides thereof via an adhesive layer, and a retardation film 2 (or retardation layer directly layered on one side of the polarizing plate 1) layered on one side of the polarizing plate 1 via an adhesive layer 2a. When inspecting defects such as foreign substances, scratches, nicks and the like existing on the polarizing plate 1, light is irradiated from an appropriate light source onto the polarizing plate 1, a reflected light image or a transmitted light image is acquired via an imaging unit such as a line sensor camera or a two-dimensional TV camera, and defect detection is performed based on the acquired image data. When inspecting the polarizing plate, the image data is acquired with an inspection polarization film interposed in the middle of a light path between the light source and the imaging unit. Normally, a polarizing axis (e.g., polarized absorption axis) of the inspection polarization filter is arranged in a state (cross Nicol) orthogonal to a polarizing axis (e.g., polarized absorption axis) of the polarizing plate 1 to be inspected. According to the cross Nicol arrangement, an all-black image is input from the imaging unit if defect does not exist, and the relevant portion does not become black if the defect exists. Therefore, defect can be detected by setting an appropriate threshold value.
In the layered film in which the optical compensation layer having retardation exists in addition to the polarizer, however, the optical axis shifts when the light from the light source passes through the optical compensation layer, and the polarizer and the inspection polarization filter will not be in a substantially cross Nicol state. As a result, the defect inspection of the polarizing plate cannot be accurately performed.
A polarizing plate inspection device disclosed in Patent Document 1 is known as a layered film (polarizing plate with protective film) defect detection device for solving the above problems. The polarizing plate inspection device is arranged with a light source arranged on one side of a polarizing plate; an imaging unit arranged on the other side of the polarizing plate and imaging a transmitted light image of the polarizing plate; and an inspection polarization filter and an inspection retardation plate arranged in a light path between the polarizing plate and the light source. Using the inspection polarization filter for converting the light from the light source to a linear polarized light, the linear polarized light is input to the polarizing plate with protective film, and defect detection is performed based on the transmitted light image. Furthermore, a retardation plate for compensating birefringence of the light caused by the protective film is arranged on the light path transmitting through the polarizing plate with protective film from the light source. The change in phase by the protective film having retardation can be canceled and the birefringence of the light by the protective film can be compensated by separately arranging the retardation plate. Furthermore, a configuration example of arranging a variable polarization optical element in which the phase angle of the light is adjustable by voltage to compensate the birefringence by the protective film that differs slightly among products is also disclosed.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-9919